Bathing preparations generally comprise an inorganic salt mixture containing sodium sulfate (Glauber's salt), borax, sulfur, sodium chloride, carbonate, etc., having incorporated therein additives such as flavors, coloring materials, plant extracts, and organic acids. The bathing preparations function not only to impart flavors or color tones to baths but also to give moderate stimulation to the skin to activate blood circulation thereby relieving fatigue and promoting metabolism.
Implicit in these bathing preparations is a bubbling bathing preparation comprising a carbonate and an organic acid. The bathing preparations of this type produce bubbles of carbonic acid gas in a bath to promote relaxation or refreshment, and to make bathing pleasant.
The organic acid which can usually be incorporated into these bathing preparations include citric acid, succinic acid, malic acid, and tartaric acid. Fumaric acid, though cheaper than these organic acids, is unsuitable for use in bathing preparations because of its sparing water solubility. Hence, fumaric acid has been used in the form of sodium monofumarate or after being pretreated with a surface active agent by spray drying.
However, sodium monofumarate, in which one of the two carboxyl groups has been neutralized with sodium, must be used in a large quantity, thus increasing the cost. Although the method of spray drying fumaric acid together with a surface active agent is effective to inhibit floating of fumaric acid, the foam on the surface of water remains. If the amount of the surface active agent is decreased, floating of fumaric acid cannot be suppressed.
It has been proposed to coat fumaric acid by spraying a water-soluble high polymer comprising a homo- or copolymer of acrylic acid or a salt thereof having an average molecular weight of from 300 to 50,000 as disclosed in JP-A-60-169436 (the term "JP-A as used herein means an "unexamined published Japanese patent application").
However, this technique has a poor production efficiency and also entails high cost. Moreover, it is very difficult to completely remove water by drying.
In order to overcome these disadvantages associated with the use of fumaric acid, it has been proposed to add, to fumaric acid, from 1 to 20% by weight of one or more of sodium carboxymethyl cellulose, polyvinylpyrrolidone and water-soluble gelatin and from 0.1 to 0.5% by weight of a surface active agent having an HLB of 7 or more as disclosed in JP-A-59-106415.
The bathing preparation obtained by this method, however, still has disadvantages in that it causes foaming on agitating a bath after it is dissolved and the foam hardly disappears.
Therefore, there has not yet been developed a satisfactory technique to produce a bubbling bathing preparation containing fumaric acid which is free from any of the above-described disadvantages.